1. Field of the Invention
The present invention relates to laminated safety glass and the manufacturing thereof, especially for automotive applications.
2. Description of the Related Art
Laminated glass is a type of safety glass that holds together when shattered. In the event of breaking, it is held in place by an interlayer, between its two or more layers of glass. The interlayer keeps the layers of glass bonded even when broken, and its high strength prevents the glass from breaking up into large sharp pieces, thereby creating a characteristic “spider web” cracking pattern when the impact is not enough to completely pierce the glass.
This safety glass is typically used as a windshield for the front window of a vehicle. These windshields are often glued into the vehicle frame by an adhesive, which necessitates protective measures against degradation of the adhesive over time, e.g. by UV light or acid rain. For the latter, a rubber seal (28 in FIG. 3) is often used to prevent moisture from reaching the adhesive. Protection against UV light is obtained by a UV light blocking border (3 in FIG. 1), which usually has a black colour.
In state-of-the-art manufacturing processes (see FIG. 4) of automotive laminated safety glass, the black UV light blocking border is applied by screen printing an enamel ink on a flat windshield shaped glass layer 42 before gravity-sag shaping in a long tunnel oven or furnace 35. In this furnace, temperatures above 600° C. are used to bend the glass layer under its own weight and simultaneously also fire the enamel ink which generally consists of 50 to 85 wt % of low melting glass frit fluxes, 10 to 40 wt % of inorganic pigments and other additive oxides, sulphides or metals. Pigments often used for black colour development include copper chromite spinel CuCr2O4 and chrome iron nickel spinel (Ni,Fe)(Cr,Fe)2O4. Typical enamels are disclosed by, for example, EP 2368859 A (FERRO CORP). Sag bending is performed by shaping the glass layers as matched pairs, in order to avoid later lamination problems in the autoclave 39.
In automobile design, improved aerodynamics and visibility require more glass surface area with more complex shapes. Also, lighter vehicle weight requirements for improved fuel efficiencies are resulting in thinner glass. As gravity-sag bending is unable to form severe bends, the press bend technology or pressure forming technology was developed. One bent glass layer at a time is formed by lower and upper moulds, as such press bend forming operations allow precise shape control, more severe and complex shapes, and faster production rates. An apparatus for press bending glass layers is disclosed in e.g. U.S. Pat. No. 5,882,370 (PILKINGTON GLASS).
During the pressing operation, the enamel is in direct contact with a press mould or cloth thereon in the hot zone of the furnace, making it vulnerable to scratches and abrasion. Lead based glass frit fluxes, such as PbO—B2O3—SiO2, ensure good durability but are today limited in use by environmental legislation.
US 2008206504 (DUPONT) discloses a process of preparing decorative safety glass comprising a coated image-bearing rigid sheet: (a) providing a rigid sheet; (b) printing an image on the rigid sheet so as to produce an image-bearing rigid sheet containing an image-bearing side; and (c) coating an adhesion promoter on the image-bearing side and over the image to produce a coated image-bearing rigid sheet containing a coated image-bearing side. It was found that printing the UV light blocking border with an inkjet ink, e.g. a UV curable black inkjet ink containing carbon black, not only caused adhesion problems but also failed to reach the desired opacity for effective UV protection of an adhesive with which the laminated safety glass is glued into a vehicle frame.
JP 2010138008 (BRIDGESTONE CORP) discloses a laminated safety glass wherein an intermediate film for decorative laminated glass can be manufactured from a resin composition containing the lubricating agents stearic acid and zinc stearate.
Hence, there is still a need for an improved laminated safety glass that can be manufactured by press bending without deterioration of physical properties, like opacity (UV light blocking), adhesive bond strength and glass strength.